All internal combustion engines generate a series of discrete exhaust pressure pulses whose basic characteristics of peak amplitude, waveshape, and repetition rate depend on the type of engine, the speed and the load. Conventional four cycle reciprocating piston engines of the type ordinarily used in automobiles exhibit a field of pulses having, from the standpoint of sound attenuation, relatively desirable characteristics in that the waveshapes are relatively symmetrical or sinusoidal, the peak to average pressure ratios are moderate, and the higher harmonic amplitudes decrease rapidly with increases in frequency. This is due to the relatively high engine RPMs and light load on those engines under normal operating conditions. In general, the exhaust gas discharged by these piston engines has been satisfactorily silenced by apparatus derived from classical methods of analysis assuming a linear sound field.
Other engines, however, such as diesel engines for trucks, generate high energy pulses which appear to be N-shaped, with extremely high peak and peak to average pressures, with very unfavorable dissymmetries causing a flat frequency-pressure response, and an unusually large number of acoustically significant notes. In the case of truck engines, the overall noise level is considerable and the nodal/anti-nodal characteristics of the exhaust system conduits virtually disappear. Thus, the usual standing wave theory of exhaust silencing is not satisfactory and truck engines have presented a significantly new exhaust gas silencing problem requiring a new approach for solution. This problem becomes more acute as engine RPM limiting devices are advanced downwardly to accommodate reductions in maximum road speeds.